Porous polymer composites are of significant interest due to their low density, large surface area, unique mechanical properties, tunable reactivity and mass transport properties. This combination of properties has potential utility in a variety of applications including catalyst supports, tissue surrogates, tissue scaffolding, controlled barrier/transport properties, blast mitigation materials and hybrid structural materials. For many of these applications, the performance of the porous polymer composites is directly linked with pore size and pore connectivity. For example, a decrease in the pore size at a constant pore volume will result in additional surface area, providing increased interaction with transported material. In addition, pore connectivity must be maximized to provide access to the entire pore volume.
A common way to produce porosity in a material is through the use of a foaming agent. Porosity created using a foaming agent results in a closed cell, where the air voids are completely encapsulated by a matrix. Closed cell porosity provides a decreased density but does not allow access to the air voids, thereby eliminating any mass transport through the material. To produce more open cell porosity using a foaming agent, the material must be engineered such that the pressure produced by the foaming agent is more than the cell wall can withstand. Foams have also been modified through the addition of a filler. However, the maximum filler concentration is often limited by the ability of the foaming agent to physically expand the composite matrix.
Porosity has also been incorporated into a polymer matrix through phase separation of the formulation. In these systems, the components have limited miscibility resulting in the constituents segregating into discrete regions prior to cure. After curing, one of the constituents can be preferentially etched resulting in a porous matrix of the other materials. Successful implementation of this approach requires that the constituent materials phase separate and an understanding of the phase separation kinetics. Dependent on the phase separation kinetics, an incorrect time between casting and curing can result in incomplete phase separation or macroscopic phase separation. An alternative method is to use covalently attached phase separating materials, i.e. block copolymers, where the phase separation length scale is controlled by the block length. However, block copolymers often have limited commercial availability, complex phase behavior, and slow phase separation kinetics that will limit their practical implementation.
Particulate fillers have also been incorporated into polymeric materials to produce porosity and modify material properties. One method is to use the particulate as a sacrificial poragen where the filler is incorporated into the material during cure and then etched away after cure to leave a void. To obtain interconnected pores, high poragen loadings are required to ensure direct contact between adjacent poragen prior to etching. At high loadings, the particulate will produce an exponential increase in the viscosity decreasing the ability to process the material. The limitation on particulate loading to remain processable has a large impact on the porosity, surface area, and mass transport achieved in the resulting porous polymer. In addition, the removal of the particulate eliminates the potential multi-functional benefits of incorporating the particulate into the porous backbone. Particulate has also been incorporated into phase separating systems where the particles are not involved in the development of porosity but remain in the porous structure after poragen removal. However, the particulate loading is still limited by processing difficulties and the particulate can alter the phase separation kinetics. As a result, it can be difficult to rapidly transition between different matrix materials. In addition, the particles are often embedded in the polymer to limit any multi-functionality through interaction with the transported material.
Porosity has also been obtained through hydrolysis and condensation of metal alkoxides to form aerogels consisting of alumina, silica, titanium dioxide and tin oxide. This concept has also been expanded to aqueous polycondensation reactions of organic precursors followed by pyrolosis to produce carbon aerogels. These materials have significant technological potential but have achieved limited application because the unique structure that provides the properties also results in the materials being fragile and brittle. A typical method of enhancing the mechanical toughness of the aerogel is to utilize a formulation that incorporates functional groups onto the particulate after hydrolysis and condensation of the metal oxide pre-cursor. The functional groups are then used to incorporate a conformal polymer coating on the structure to enhance the mechanical toughness of the aerogel. While the material does become more mechanically robust the process eliminates access to the particulate surface. In addition, adapting this approach to other particulates or chemistries can be challenging and may reduce the functionality of the particulate.
Porous polymer composites also have utility in hemorrhage control related products. Specific to hemorrhage control, treatment delays in combat environments can lead to infection and massive blood loss, resulting in severe consequences including death or limb amputation. In fact, hemorrhage from wounds is the leading cause of death on the battlefield. A major challenge for treatment procedures is to develop materials and delivery mechanisms that can address the various types of wounds encountered during deployment. Examples include punctures from sharp devices, bullets, or shrapnel; multiple punctures from secondary shrapnel associated with an indirect impact; massive body and limb trauma from Improvised Explosive Devices (IEDs); severe cuts and lacerations; or even large scale skin and muscle damage. In addition, wounds that are not treatable with tourniquets or compressive wound dressings such as complex groin or torso injuries are of increased concern. Due to these complications with combat casualty care, a critical need exists for therapeutic materials that are delivered locally and immediately to treat different types of battlefield wounds, until more comprehensive medical treatment is available.
Current products related to hemorrhage control fall into three general categories including: Poly-N-acetyl glucosamine (PNAG, chitin or chitosan) based dressings; zeolite or starch based powders; and fibrin based dressings. These materials generally work by absorbing water from the blood and allowing blood clotting factors to concentrate in the wound. However, additional factors such as adhesion to the wound, attraction of blood platelets, and delivery of blood clotting factors can also contribute. While each product has demonstrated effectiveness with specific types of injuries, numerous limitations hinder broad utility in combat environments including: lack of wound conformity, inability to penetrate deep wounds with complex geometry, difficulty treating large or multiple wounds, post-treatment removal of particulates, uptake of particles in arteries, cost-effectiveness, and delayed deployment. A clear need still exists for materials and treatment methods for hemorrhage control that are effective in deep wounds, stop arterial bleeding, can conform to complex wound geometry, are easily applied, and cost-effective.
Therefore, the inventors have provided improved porous particulate-loaded polymer composites and methods of preparing such porous particulate-loaded polymer composites.